1. Field of the Invention
The present invention relates to an ATM communication system containing plural ATM communication terminals and an ATM switch, and more particularly to an ATM communication system which is suitable for use when the transmission rate of a transmission path for connecting an ATM communication terminal and an ATM switch is higher than the transmission rate of data to be transmitted/received by each ATM communication terminal and the transmission path has extra capacity.
2. Description of Related Art
Recent developments of digital information reducing technologies have promoted international standardizing organizations such as International Organization for Standardization (ISO), International-Telecommunication Union-Telecommunication Standardization Sector (ITU-T), to advance the works for standardization of multimedia coding techniques to reduce multimedia information such as images, sound, etc. and actually have completed some of this work. Of these multimedia coding techniques, a system known as xe2x80x9cMPEG-2 (Moving Picture Experts Group-2) which is to be applied to broadcasting and storage media has been known. MPEG-2 enables transmission of images of the current television broadcasting quality at a transmission rate of about 6 to 10 Mbit/s. It also enables transmission of HDTV (High Definition Television) signals having a resolution of twice or more of current televisions in horizontal and vertical directions to be transmitted at a transmission rate of about several tens Mbit/s.
B-ISDN (Broadband-ISDN) which can transmit signals at a transmission rate of 155.52 Mbit/s has been known as a communication infrastructure. ATM (Asynchronous Transfer Mode) has been used as a system for transmitting multiplexed signals by using the B-ISDN. ATM is suitable for use in handling multimedia data of various bit rates which range from low bit-rate data such as audio data to high bit-rate data such as image data. According to the ATM system, a signal such as a video signal, an audio signal, a data signal or the like is disassembled into 48-byte unit signals. Each of the 48-byte unit signals thus obtained is added with a header (describing a destination or the like) of 5 bytes and then transmitted. The signal of 53 bytes in total, which is obtained by adding the 48-byte unit signal with the header of 5 bytes, is called a cell.
In ATM, when information to be transmitted does not reach 155.52 Mbit/s, null information called a null cell is transmitted to match the transmission rate to the transmission rate of the transmission path. In this case, a signal comprising a combination of a cell containing data and a null cell, that is, a cell which is matched with the transmission rate of the transmission path is hereinafter referred to as xe2x80x9cATM signalxe2x80x9d.
In the conventional technique, when signals reduced by MPEG-2 described above are transmitted in ATM by using B-ISDN, the ATM signal of 155.52 Mbit/s contains cells having effective data of only 6 to 10 Mbit/s, and thus the residual bits of 145 to 149 Mbit/s are allocated to null cells.
Further, in the conventional technique, when many ATM communication terminals are mutually communicated to one another through an ATM switch, a line 12 must be provided between the ATM switch 5 and each of the plural ATM communication terminals 13, as shown in FIG. 12. Therefore, work for laying new lines must be performed with new ATM communication terminals, and the cost for establishing the system is increased.
An object of the present invention is to provide an ATM communication terminal and an ATM communication system which can solve the above problem and efficiently use the transmission capacity of a communication circuit.
Further, another object of the present invention is to provide an ATM communication terminal and an ATM communication system which can reduce the number of lines disposed between an ATM switch and each of ATM terminals constituting the system irrespective of the number of the terminals.
In order to solve the above problem, an ATM communication terminal according to the present invention has a function of disassembling multiplexed signals obtained by multiplexing data into cells of a fixed length, adding each of the cells with a header containing destination information and transmitting/receiving the cells through a communication line, and comprises a discriminator for referring to the destination information of the cells which are received through the communication line to identify a cell addressed to the communication terminal which received the cell (or, hereinafter referred to as xe2x80x9cself communication terminalxe2x80x9d) and a cell addressed to another communication terminal from each other, a multiplexed-signal generator for multiplexing data to be transmitted from the self communication terminal to generate multiplexed signals, a cell forming unit for forming the multiplexed signals generated in the multiplexed signal generator into cells, and a cell re-multiplexer for re-multiplexing the cell generated from the multiplexed data to be transmitted from the self communication terminal and the cell addressed to another communication terminal which is discriminated in the discriminator.
Further, a communication network is formed in a ring shape among plural communication terminals, and cells which are received through the communication network by a communication terminal concerned (self communication terminal) and are to be transmitted to (i.e., are addressed to) other communication terminals are multiplexed with cells obtained by forming data to be transmitted from the communication terminal in question into cells, and then the cell-multiplexed cells are transmitted to a transmission path, whereby the cell-multiplexed cells transmitted from the plural communication terminals are allowed to make the round on the ring.